| dc.description.abstract | Active Galactic Nuclei (AGN) are among the high luminosity (1011 − 1014 L⊙ ) sources in the
observable Universe, believed to be powered by the accretion of matter onto supermassive
black holes (SMBHs; 105 − 1010 M⊙ ) at the centres of galaxies. The energy released from
AGN could affect their host galaxies and their large-scale environment through a process called
feedback. AGN feedback is generally invoked in galaxy formation models and simulations
to explain the observed correlation between the mass of SMBHs and various host galaxy
properties. A viable feedback mechanism in AGN is outflows. The driving force behind such
outflows, though debated, can have an impact on the interstellar medium (ISM) of their hosts by
inhibiting (negative feedback) or enhancing (positive feedback) star formation (SF). In massive
galaxies, there are evidences that AGN regulate SF in their hosts via the jets, injecting energy
into the gaseous halos and regulating the cooling of gas onto the galaxies. Recent observations
available from a limited number of sources point to AGN having both positive and negative
impacts on the SF characteristics of their hosts. However, the nature and details of the impact
AGN have on the evolution of their host galaxies remain controversial and uncertain. Also,
AGN affecting SF was not known in dwarf galaxies before a couple of years ago. But, only very
recently, there are observational evidences of dwarf galaxies hosting AGN, thereby challenging
theoretical models that generally invoke supernovae feedback in dwarf galaxies. In this thesis,
we carried out a systematic investigation on a sample of AGN to find clues to (a) what triggers
outflows in AGN, (b) the impact of AGN on the SF of their hosts and (c) the nature of AGN
feedback in a dwarf AGN hosted by an intermediate-mass black hole (IMBH; 104 − 105 M⊙ ).
Towards this, we utilized imaging data from the Chandra X-ray Observatory, the Ultra-Violet
Imaging Telescope (UVIT) aboard AstroSat, the Hubble Space Telescope (HST), the Himalayan
Chandra Telescope (HCT), and the Very Large Array (VLA). Additionally, we incorporated
spatially resolved spectroscopic data in the optical and infrared bands from Gemini and SDSS
(MaNGA), as well as millimetre data from ALMA. The findings of this thesis are summarized in
four chapters, as described below.
First, we aimed to understand the driver of ionized outflows in AGN. For this, we carried out a systematic investigation of the [O III]λ 5007 emission line on a sample of AGN, consisting of radio-detected and radio-undetected sources using MaNGA and Faint Images of the Radio Sky at Twenty cm (FIRST) survey data. We found radio-detected sources to show an increased outflow detection rate compared to radio-undetected sources. We noticed a strong correlation between outflow characteristics and bolometric luminosity in both samples, except that the correlation is steeper for the radio-detected sample. Our findings suggest (a) ionized gas outflows are prevalent in all types of AGN, (b) radiation from AGN is
the primary driver of ionized gas outflows, (c) radio jets play a secondary role in enhancing the gas kinematics over and above that caused by radiation and (d) SF is quenched in the very central regions of the galaxies in the sample studied due to AGN activity.
Then, we investigated the impact of AGN on the SF characteristics of their hosts. This was done by mapping the star-forming regions in galaxies hosting AGN and looking for any correlations between the deduced SF and AGN properties. It is natural to expect that the influence of the central AGN on their hosts could decrease from the centre to the outskirts
of the galaxies. In the nearby Universe, Seyfert and LINER type AGN are ideal targets to investigate this connection as the resolution offered by ground-based imaging observations will enable one to probe SF on scales from a few hundred parsecs to a few tens of kpc. Most studies of SF in Seyfert galaxies have been conducted in the optical, IR, or radio wavelengths using ground and space-based observatories. While there have been a few studies in the UV band using Galaxy Evolution Explorer (GALEX), the resolution provided by GALEX is often insufficient to resolve SF on parsec scales. A limited number of studies have used the HST, which offers the capability to resolve parsec-scale structures, but it has a restricted field of view, making it observationally expensive to study a large number of sources comprehensively.
To address this gap, we undertook a systematic investigation of the SF properties of Seyfert galaxies using UVIT. From an investigation of the SF characteristics on a sample of eight AGN, using UVIT observations, we found a positive correlation between the total surface density of SF and extinction. For five sources, we found a gradual decline of both the surface density of SF and extinction from the centre to the outer regions. We found the ratio of the star formation rate (SFR) in the nuclear region to the total SFR to be positively correlated with the Eddington ratio. This points to the influence of AGN in enhancing the SF characteristics of the hosts. This impact is found to be dominant only in the central few kpc regions with lesser effect on the larger scales probed in this thesis.
Afterthat, we investigated the nature of AGN feedback on a dwarf AGN, NGC 4395. This is important, as dwarf AGN are believed to be powered by the lower mass end of SMBHs. In the literature, theoretical studies on the regulation of SF in dwarf galaxies have been attributed to radiation from young stars and supernova explosions. However, recent theoretical
studies do indicate that AGN could play a significant role in regulating SF in dwarf galaxies. Observationally, there is evidence of AGN feedback operating in dwarf galaxies covering angular sizes smaller than about an arcmin. Therefore, detailed studies on SF characteristics of dwarf galaxies hosting AGN are needed, firstly, to characterize their SF properties and secondly, to find evidence of the feedback process, if any, in them. Using data from UVIT, we identified a total of 284 star-forming regions extending up to a distance of 9 kpc. Of those, 120 regions were also identified in the Hα continuum subtracted image. The detection of fewer star-forming regions in Hα is attributed to the lower spatial resolution as well as the shallowness of the Hα image relative to UV. On inspection of the spatial distribution of the surface density of SFR in
the UV, we found three star-forming regions near the AGN that have a high surface density of SFR. One out of the three star-forming regions in the UV is also found to have a high surface density of SFR in Hα and younger age. This could possibly hint at positive feedback from the AGN. At 1.4 GHz, we found a few complexes having enhanced radio emission. These complexes contain a larger number of star-forming regions, with the majority of them having higher SFR. These complexes are known to host supernova (SN) remnants. The star-forming regions in these complexes have higher SFR in Hα and 24 µm, compared to other star-forming regions, arguing for SN-induced SF.
Then, we aimed to understand the effect of jets on the ISM. There is hardly any observational evidence of jet−ISM interaction and its impact on the host galaxies of AGN on parsec scales. This was investigated on the dwarf AGN, NGC 4395, powered by an IMBH. Using high-resolution observations at 15 GHz from the VLA and the HST, we found evidence
of radio jet−ISM interaction on the scale of an asymmetric triple radio structure of ∼ 10 pc size. The high-resolution radio image and the extended [O III]λ 5007 emission, indicative of an outflow, are spatially coincident and are consistent with the interpretation of a low-power radio jet interacting with the ISM. The spatial coincidence of molecular H2 λ 2.4085 along
the jet direction, the morphology of ionized [O III]λ 5007, and displacement of the CO(2−1) emission argues for conditions less favourable for SF in the central ∼ 10 pc region .
Finally, we provide a summary of the thesis work, highlighting its key findings and significance. Additionally, we discuss the unique contributions of this research and outline potential future directions that could further enhance our understanding of the topic. | en_US |
